Consumers of cellulosic sheet material such as tissue paper products have long been known to desire such products to feel soft. Another desired physical characteristic of tissue paper products is strength. Just as a strong tissue paper product that is hard, or unpleasing to the touch, is generally disfavored by consumers, so too are soft tissue paper products that lack sufficient strength. Therefore, for many years, there has been extensive research in the field of tissue papermaking to discover methods of producing soft yet strong cellulosic sheets. However, a recurring problem is that the physical phenomenon upon which the strength of cellulosic sheets depends--the formation of hydrogen bonds between adjacent fibers--is also the factor that detracts from the softness of such sheets.
One prior art method of imparting softness to cellulosic tissue paper sheets is to apply work to the sheets. For example, at the end of most conventional tissue papermaking processes, the sheets are removed from the surface of a thermal drying means, such as a Yankee drum, by creping them with a doctor blade. Such creping breaks many of the inter-fiber hydrogen bonds throughout the entire thickness of the sheet. However, simple creping produces tissue paper that is neither as soft nor as strong as is desirable.
The prior art therefore turned to treating cellulosic tissue paper sheets or their cellulosic web precursor, with chemical debonding agents that disrupt the inter-fiber hydrogen bonds. See, e.g., U.S. Pat. Nos. 4,144,122; 4,372,815; and 4,432,833.
For example, U.S. Pat. Nos. 3,812,000; 3,844,880; and 3,903,342 disclose the addition of chemical debonding agents to an aqueous slurry of cellulosic fibers. Generally, these agents are cationic quaternary amines such as those described in U.S. Pat. Nos. 3,554,862; 3,554,863; and 3,395,708. Other references disclose adding the chemical debonding agent to a wet cellulosic web. See, U.S. Pat. No. 2,756,647 and Canadian Pat. No. 1,159,694.
These methods have been found to suffer from a serious drawback. The addition by the prior art of the chemical debonding agent to an aqueous slurry of cellulosic fibers or to a cellulosic web with a high moisture content results in the distribution of substantial quantities of the chemical debonding agent throughout the entire thickness of the cellulosic tissue paper sheet. See, e.g., the paragraph bridging columns one and two of U.S. Pat. No. 2,756,647. This causes an unacceptable decline in the strength of the sheet. Furthermore, from the use of strong acids to acidify the chemical debonding agent, such as disclosed in Canadian Pat. No. 1,159,694, are derived environmental and economical drawbacks.
Another problem with chemical debonding agents in general, and cationic quaternary amines in particular, is that they substantially interfere with the adhesive/release agent combination normally employed to obtain proper adhesion of the cellulosic web precursor to the Yankee drum thermal drying means.
In conventional tissue papermaking processes, a cellulosic web is formed; the web is subjected to non-thermal eewatering, such as by a series of vacuum boxes or vacuum pressure rolls; the dewatered web is adhered to a thermal drying means, such as a Yankee drum, and dried; and the dried web is creped from the surface of the Yankee drum by a doctor blade. Adhesion of the web to the Yankee drum/creping surface is accomplished by contacting the web with an adhesive that is usually sprayed upon that part of the rotating Yankee drum that is not yet in contact with the advancing web. Further, to the adhesive is usually added a release agent that prevents the web from adhering too strongly to the Yankee drum. The cationic quaternary amines that function as chemical debonding agents are known to act as release agents. Thus, the use of cationic quaternary amines as chemical debonding agents created release problems.